I have a nagging question that just hasn't yet been answered to my satisfaction. Why is roach in a mainsail a good idea?

I feel satisfied with the arguments of battens (full and partial) vs battenless, but the physics and benefits of a lot of mainsail roach elude me. I had thought that the roach was probably for rule beating purposes and would allow you to get a better PHRF rating by having a low measured luff but more sail area.

The problem with that theory is that pretty much all open class boats have lots of roach, especially at the head of the sail. There must be some specific benefit of roach besides "more sail area". I've also noticed that the wings of airplanes don't go to a point like a triangle. They have "roach" too.

So what does it do? Lots of roach seems to be at odds with the ultra high aspect modern rigs that dominate the races. If high aspect and all luff and no foot is the ideal, then why the roach?

The roach allows the sail to form a curved surface, creating an airfoil, and threrefor generating "lift", just like the wing of an airplane, or the rotor blades of a helicopter. The physics involve Bernouli's Principle, best illustrated with... an illustration

The Luff and Loach of the sail are equivalent to the leading and trailing edges of an airlplane wing, and the straight lind distance between those is the "chord". (also, the aeronautic equivalent of "roach" is "camber")

The air flowing past the sail has to flow faster on the convex side of the airfoil, which results in lower pressure, which is called "lift". The"lift" is more or less perpendicular to the chord, so of course in the case of the sail as airfoil, the "lift" pulls the sail and boat forward, rather than up.

If you think about sailing on any point higher than a beam reach, you can see that it is the lift generated by airflow over the curved shape of the sail, and not increased sail area that is propelling the boat forward. It could be argued that increased sail area alone is only really advantagous when running downwind, wing and wing.

Roach DOES make more sail area. My maxed out per phrf main is 195#, vs 180 for the basic supplied main, actual triangle measure is 161 sq ft.

So orach to a degree, and maybe at one time was a rule beater, but now, you have to measure the foot, luff, pluse the measure from luff to the back edge of the main af the half, 3/4 and 7/8 of the way to the top IIRC, so that the total roach amount gets included. If you go over a certain % of the base foretriangle, you get penalized, ie need to go faster to finish. If my main was around 100#, I would get a 3 sec penalty per mile, so instead of a 192, I would be a 189 in phrf speak. I could go bigger yet, and get more deducts too......some classes this extra is built into the base ratings. If one wer to go smaller, say 180 as I was, I get a 3 sec credit, and was a 195 with the smaller main.

The same happens with jib/genoa and spin sizes too, while you can go bigger per say, you may take a hit faster from the base if you go too large, or go smaller, you get credits. Many times the credits frankly are not in the end worth taking. As you go double or triple or more the miles per second credit you get. Fixed props are this way, the 9 or 12 sec credit vs a folder is not worth it if you race frequently! you gain 20-30 secs a mile with a folder vs a fixed.

Aerodynamic theory explains that the more perpendicularly the airflow crosses or leaves the trailing edge the more efficient the total aerodynamic effect.
A roach extends the sail’s area in the upper section so that this area more approximates or approaches the above. (and because it more rapidly transitions the ‘tip’ geometry toward that desired 'perpendicular to exit flow' geometry)

Iceboats carry radically raked masts to approximate the same - leech almost dead perpendicular to the horizontal.

The latest modern racing rigs have extended 'flat head' mainsails for the same reason --- more perpendicular flow ‘off’ of the leech; for more maximized efficiency and more optimized lift. The more efficient the mainsail, …. the jib becomes more aero-efficient, too … when sailing ‘aerodynamically’.

The absolute master of aerodynamic design for similar wind speeds, does the same ---- the albatross’ wing.
(So don’t be at all surprised when someday in the future you might even see mainsails with extended ‘primary feathers’ along the top of that big ‘square-ish’ & ‘flat top’ mainsail.)

-----------
BTW ... fabric sails dont have 'thickness', and therefore much of what you learned from sailing books about what causes ‘lift’ doesnt happen in that manner, and yet the Wright Brothers original wings were essentially sails with essentially NO thickness …… hmmmmmm.
More confusing to sailors, and some pilots, some of the airflow circulates 'around' a wing/sail (even ‘thick’ wings) ... there is a flow 'component' on the 'windward side' that is going --- FORWARD, due to the circulation flow ‘around’ the wind/sail!
Aerodynamics is definitely NOT an intuitive science, and your (US) high school 'science' teacher's explanation of 'lift' was almost DEAD WRONG

The roach allows the sail to form a curved surface, creating an airfoil, and threrefor generating "lift", just like the wing of an airplane, or the rotor blades of a helicopter.
The Luff and Loach of the sail are equivalent to the leading and trailing edges of an airlplane wing, and the straight lind distance between those is the "chord". (also, the aeronautic equivalent of "roach" is "camber")

The air flowing past the sail has to flow faster on the convex side of the airfoil, which results in lower pressure, which is called "lift". The"lift" is more or less perpendicular to the chord, so of course in the case of the sail as airfoil, the "lift" pulls the sail and boat forward, rather than up.

If you think about sailing on any point higher than a beam reach, you can see that it is the lift generated by airflow over the curved shape of the sail, and not increased sail area that is propelling the boat forward. It could be argued that increased sail area alone is only really advantagous when running downwind, wing and wing.

Does that help?

Sorry to debunk that tired old theory, but sails, like airplanes wings do not work because of our friend Bernoulli. Instead it is the Coanda effect (

Basically, a fluid attaches to a curved surface and is redirected off that surface providing drive. (simple explanation) Think of what happens when you turn on your kitchen faucet. Now, take a simple tablespoon and hold it vertically w/ the underside of the spoon towards the running water. Slowly move the spoon towards the water. When you actually engage the water what happens? (rhetorical question) The water attaches to the back of the spoon and is redirected out of the flow and sprays out into the sink. That's what happens w/ a foil, be it a wing, a sail or a keel. Subtle difference from Bernoulli. And why a jet fighter that has almost no curvature to its wing can fly upside down. It just has a higher angle of attack and thus forces the air downward. Sorta like what happens when you stick your arm out the window of a moving car w/ you hand held flat. If you tilt your arm so your hand is acting like a wing, it wants to rise.

But, back to the OP question. Roach, as has been pointed out adds more sail area providing more drive. And yes, the further up the mast you can add sail area the better because of surface friction. That's why all the hot race boats have the square top mains; puts more drive at the top of the mast where there is better wind speed and apparent wind angle.

The "more sail area" argument just wasn't holding water with me. Many new boats that have no rule limitations on design are building roachy heads into their mains. If it was truely just sail area, then they'd make the mast taller right?

Your explanation may also explain something I read in an article by Carol Hasse on small jibs whereby she was saying that if you have an extremely long luff with very little LP you end up with a lot of narrow sail at the top of the luff that does little to create lift but does a lot to induce heel.

On my boat I have a VERY low aspect mainsail (P=42, E=17), which would mean that I have a larger angle at the head of my sail than I high aspect rig. Would I benefit (in theory) from battens at the top, or is my rig low enough aspect that I already have enough sail area at the head? Is there a painful formula that I can use to figure out how much sail area should be at the head or how much roach would be idea for my sail?

Square top mains provide a lot more roach, to the extreme. Sail area up high is good, b/c that's where there is more breeze. Also, triangles have some tip vortexes that are slow. Square top mains reduce the drag from these.

The "more sail area" argument just wasn't holding water with me. Many new boats that have no rule limitations on design are building roachy heads into their mains. If it was truely just sail area, then they'd make the mast taller right? ...
******* yes, probably thats whats going on in the high class venues. A taller mast would increase the heeling moment -- its like supply and demand ... needs 'balance'. I strongly feel the roachy heads are for sail efficiency increase .... and the more SA thats aloft the better the upwash is forming waaay out in front of the boat, more SA up top the more 'upwash' aloft. With strictly triangular tops, the upwash directly in front of the top is less, simply because there's less sail area 'coming forward' to create that upwash. all this for 'upwind' sailing.

Your explanation may also explain something I read in an article by Carol Hasse on small jibs whereby she was saying that if you have an extremely long luff with very little LP you end up with a lot of narrow sail at the top of the luff that does little to create lift but does a lot to induce heel.
******* the preponderance of the 'lift' comes from the luff section, the 'trailing sections' through to the leech make the luff section more efficient (simplistically but NOT in direct proportion to 'length' ... and you can see the scarcity of large overlapping jibs on high end boats these days, too); making the luff longer increases the heeling moment. see sketch below.

On my boat I have a VERY low aspect mainsail (P=42, E=17), which would mean that I have a larger angle at the head of my sail than I high aspect rig. Would I benefit (in theory) from battens at the top, or is my rig low enough aspect that I already have enough sail area at the head? Is there a painful formula that I can use to figure out how much sail area should be at the head or how much roach would be idea for my sail? Sail what you have, or at least 'fill in' all the space from the backstay towards forward with roach.
****** But your boat has a staysail and if flown 'correctly' when upwind (and usually above 7kts) the staysail 'can' become a forward 'extension' of the mainsail system. Here's how: http://www.arvelgentry.com/magaz/The...e_Head_Rig.pdf. If your boat isn't tender after it initially 'hardens up' on a beat --- easily heels over to a quite constant angle and no more; then a proper 'matching' of a new mainsail with a full 'shoulder' (top panels with extra draft) would probably be the best way to go, will reduce your light wind performance a bit but could add a lot more 'power' (not speed) to the top portion of the mainsail. This is an individual sail design that you're not going to get from a 'stock sail' loft AND you're going to need a sailmaker who actually 'deals' with such design changes. A sail with lots of 'shoulder' will develop lots of extra power and the boat needs to be somewhat stiff or in racing a crew that can hike waaaaay out. ... or a 'blend' between plain vanilla and full shouldered.
An alternative to full-shoulder, is to 'hook up' the leech by over tensioning the mainsheet until the second batten 'just' points above the boats center line, then 'very' slightly ease the traveller until that #2 batten is parallel to the CL., such can create a lot of increased 'draft' up top (and the mid section and foot, so dont overdo it) To do this the sail has to be correctly/perfectly raised (if dacron) and the luff boltrope cant be 'shrunken' (an age problem).

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